Currently, the 3rd generation partnership project (3GPP) is standardizing a feature called Narrowband Internet of Things (NB-IoT) for satisfying the requirements of Machine Type Communication (MTC) applications, while maintaining backward compatibility with the current Long Term Evolution (LTE) radio access technology. NB-IoT transmissions may occur in-band of a wideband LTE transmission, within a guard band of a wideband LTE transmission, or in standalone spectrum. Regardless, communication of control information and payload data in a NB-IoT environment proves challenging because it may be necessary for NB-IoT devices to operate in environments that exhibit very low signal to noise ratios (SNRs).
To support wireless communication in such environment, each NB-IoT base station (eNB) and user equipment (UE) may be configured to repeat transmission of control and data blocks (e.g., transport blocks or other subsets of packet data) to one or more destination NB-IoT devices in both the uplink (UL) and downlink (DL). On the receiving side, data from each repetition is soft-combined before decoding. The number of repetitions will be configured per UE (and further varies per physical channel).
Link simulations show that up to 24 repetitions might be necessary to achieve the targeted gain (up to 164 dB coupling loss) for some channels. Note that repetition is done per packet spanning over several one-millisecond (ms) transmission time intervals (TTIs). For instance, to reach 164 dB, 24 repetitions of a 776 bit packet spanning over 9 ms requires a 216 ms transmission/reception time in total. Therefore, the time constraints imposed by such systems are a significant challenge.
In addition to these time constraints, NB-IoT systems operate in a narrow frequency bandwidth, utilizing only a single physical resource block (PRB) of size 180 KHz, which is divided into several subcarriers. This bandwidth reduction allows for less complex hardware in the NB-IoT UEs, thereby lowering associated manufacturing costs. For frequency division duplexing (FDD) communication (i.e., the transmitter and receiver operate at different carrier frequencies), only half-duplex mode needs to be supported by the UE. The significantly lower complexity of the UEs (e.g. only one transmission/receiver chain) demands transmission repetition for communication integrity in low SNR scenarios, and even means that repetition might be needed also in normal or robust coverage scenarios. Further, to alleviate UE complexity, the working assumption is to have cross-subframe scheduling. That is, a transmission is first scheduled on an Enhanced Physical DL Control Channel (E-PDCCH or “NB-PDCCH”) and then the first transmission of the actual data on the Physical DL Shared Channel (PDSCH) is carried out after the final transmission of the NB-PDCCH. Similarly, for uplink (UL) data transmission, information about resources scheduled by the network and needed by the UE for UL transmission is conveyed on the NB-PDCCH and then the first transmission of the actual data by the UE on the Physical UL Shared Channel (PUSCH) is carried out after the final transmission of the NB-PDCCH.
Thus, for both cases above, NB-IoT does not support simultaneous reception of control channel signals and reception/transmission of data channel signals at the UE. In other words, due to the simplicity of the NB-IoT UEs, only one link and channel is supported at any time. Although this simplicity attempts to further the goal of reducing the cost of NB-IoT UEs, it threatens to wastefully consume scarce radio resources and starve some NB-IoT UEs from service.